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Calculating Force

  1. Jun 30, 2016 #1
    A cyclist is attached to a rope which is looped over a pulley (disc of mass 10kg and radius 0.5m). The rope is then
    attached to a hanging block of mass 5kg. What force must the cyclist generate such that the block will move with an
    acceleration of 2.25 m/s2. Let’s make the assumption that only one wheel of the bicycle is necessary to generate the
    force (we can ignore the front wheel) and that all the force will manifest itself in rolling friction. The mass of the
    bicycle and the cyclist is 90kg. The surface has a coefficient of static friction of 0.65 and kinetic friction of 0.25.
    mass of a wheel is 700g
    diameter of a wheel is 622mm

    2. I am having a difficult time drawing out the FBD for each piece. I know from summing the forces on each piece you can calculate the force the wheel needs to generate .

    3. I know for the block there is tension in the rope pointing up and mg pointing down. I know the disk is rotating clockwise and that it has some torque. I also know that there is friction on the bike along with the normal force and weight. The FBD of the disk is confusing me along with the problem.
  2. jcsd
  3. Jun 30, 2016 #2
    Can you draw the figure?
  4. Jun 30, 2016 #3


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    I assume the rope is horizontal from pulley to cyclist, and that the cyclist is on a horizontal road.
    For the pulley, just treat the tensions as two forces applied at the perimeter, one vertically down to the mass and one horizontal to the cyclist. There will also be a force on the axle from the pulley's support, but you don't need to worry about that since you only care about the angular acceleration equation.
    Post what equations you can.
  5. Jun 30, 2016 #4
    I know alpha=T/I So im guessing the force moving the box is the torque in the wheel of the bike.
  6. Jun 30, 2016 #5
    I also know Icom of a disk is 1/2Mr^2
  7. Jun 30, 2016 #6
    Confused on where to go from here
  8. Jun 30, 2016 #7


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    What equation relates torque and angular acceleration of the pulley/flywheel?

    What equation relates force, torque and a dimension of the pulley/flywheel?
  9. Jun 30, 2016 #8


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    We're not discussing the bike yet. I was replying to your saying you were stuck on the FBD for the pulley. When drawing an FBD for a rigid body, you work only with the forces acting directly on that body. For the pulley, there are just three: the two tensions (which will be diffeent) and the force on the axle from the support.
    Create unknowns for the forces as necessary. In terms of those, what torques act on the pulley?
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